Generally, a video receiving apparatus is an apparatus for receiving and displaying a video program transmitted from a broadcasting station through air, such as a television, or a video tape recorder (hereinafter, referred to as a "VTR"), etc.
Accordingly, a small-sized processor that has recently been developed and is commonly used in televisions or VTRs comprises amplitude of additional functions in order to increase the efficiencies of the processor in controlling the televisions and the VTRs as well as to accommodate the viewer's desire to have greater commands of the processor.
Accordingly, the video receiving apparatus has been converted so that the tuning of a broadcasting frequency for a broadcasting channel selected by a viewer is automatically carried out by the processor.
However, up to now, the automatic frequency tuning method used in the video receiving apparatus has a problem in requiring an extensive amount of time to execute the frequency tuning process, which will be described with reference to the attached drawings FIGS. 1 to 3.
FIG. 1 is a flow chart of a conventional automatic frequency tuning method, FIG. 2 is a state diagram of the frequency tuning according to the flow chart shown in FIG. 1, and FIG. 3 is a block diagram of a VTR, which is a diagram of the circuit for carrying out the flow chart shown in FIG. 1.
For convenience, the circuit shown in FIG. 3 will be described. A microcomputer 300 controls and processes the system. A key input means 310 which may be a keyboard, or a remote controlled receiver, etc. receives a key input data representative of an order for the function selected by a viewer, information about channels, and any other needed control information, and then transfers the key input data to the microcomputer 300. After selecting an arbitrary radio frequency signal among a plurality of radio frequency (hereinafter, referred to as "RF") signals receiving through a RF converter 350 under the control of the microcomputer 300, a tuner 320 frequency-converts the selected radio frequency signal and supplies the frequency-converted intermediate frequency signal to a demodulator 330. Then the demodulator 330 demodulates the intermediate frequency signal supplied from the tuner 320, supplies the demodulated video and audio signals to a signal processor 340, and generates an automatic frequency tuning signal having different logic states according to the magnitudes of the frequency of the intermediate frequency signal and a reference demodulation frequency, and then supplies the automatic frequency tuning signal to the microcomputer 300. The signal processor 340 records video and audio signals entering from the demodulator 330 on a recording medium or reproduces the video and audio signals recorded on the recording medium to be supplied to the RF converter 350. The RF converter 350 modulates the video and audio signals entering from the signal processor 340 into radio frequency signals of the frequency band corresponding to television channels 3 or 4, to be output through a connector OPT, and also outputs the radio frequency signal, received through an antenna ANT, through the connector OPT according to a television/VTR mode selection switch, or supplies it to the tuner 320.
A flow chart shown in FIG. 1 is carried out by the microcomputer 300 to be described with reference to the circuit shown in FIG. 3 and the state diagram of the tuning frequency shown in FIG. 2 as follows.
When the key input data for a channel information selected by a viewer is received from the key input means 310, the microcomputer 300 initiates a frequency search mode to enable a flag X assigned in one of its registers as "0" to set to a high frequency search mode such as 20 shown in FIG. 2, and initiates a frequency increase number N stored in one of its registers as "0" (in step 100).
After the processing step 100, the microcomputer 300 reads out a reference search frequency f.sub.o for channel information corresponding to the key input data among reference search frequencies for respective channel information stored in its ROM, and then sets the read reference search frequency f.sub.o as a reference tuning control frequency f.sub.to, and then supplies the set reference tuning control frequency with a strobe signal of a low logic state to the tuner 320 according to a clock pulse train (in step 101).
After carrying out step 101, the microcomputer 300 checks whether the frequency increase number N stored in one of its registers is equal to a limited increase number 32 (in step 102).
When the frequency increase number N is not equal to the limited increase number 32 in step 102, the microcomputer 300 adds "1" to the frequency increase number N (in step 103).
After processing step 103, the microcomputer 300 stores a logic state of the automatic frequency tuning signal supplied from the demodulator 330 in its RAM (in step 104).
After carrying out step 104, the microcomputer 300 multiplies the frequency increase number N by a unit frequency of 0.0625 MHz, and then adds the product to the reference tuning control frequency f.sub.to to set a tuning control frequency f.sub.t, and then supplies the set tuning control frequency to the tuner 320 (in step 105).
After carrying out step 105, microcomputer 300 checks whether the logic state of the automatic frequency tuning signal entering from the demodulator 330 is equal to that of the automatic frequency tuning signal stored in its RAM, and if they are different from each other, a completion of the tuning process is determined and the automatic frequency tuning operation is completed. On the other hand, if they are equal, the microcomputer goes back to step 102 (in step 106).
And when the frequency increase number N is the limited increase number 32 in step 102, the microcomputer 300 checks whether the frequency search mode flag X assigned in one of its registers is set to "1", thereby determining whether the low frequency search mode has been set (in step 107).
When the frequency search mode flag X is reset to "0" in step 107, i.e., when it is a high frequency search mode such as 20 of FIG. 2, the microcomputer 300 sets the frequency search mode flag X to "1", so as to set a low frequency search mode such as 21 of FIG. 2, and sets the set reference tuning control frequency f.sub.to, which has been set in step 101, to be smaller than the reference search frequency by 2 MHz, and initiates the frequency increase number N as "0", and then goes back to step 105 (in step 108).
Inversely, when the frequency search mode flag X is set to "1" in step 107, i.e. when a low frequency search mode is set, the microcomputer 300 sets the reference search frequency f.sub.o as a tuning control frequency f.sub.t to be supplied to the tuner 320, and then completes the automatic frequency tuning operation (in step 109).
Thus, as indicated by the flow chart shown in FIG. 1, the search mode of a frequency band lower than the reference search frequency f.sub.o corresponding to a channel by approximately 2 MHz is set in steps 100 and 101, and a broadcasting frequency for the channel is searched in the frequency band within 2 MHz from the reference search frequency according to the set search mode in steps 102 to 106, and when the broadcasting frequency can not be found in the frequency band within .+-.2 MHz with reference to the reference search frequency, the reference search frequency is fixed and set as a tuning control frequency so as to complete the tuning operation in step 109.
As described above with reference to FIGS. 1 to 3, the conventional automatic frequency tuning method sets low and high band search modes, and carries out a search process of 32 steps of increments or decrements from the reference search frequency f.sub.o to each limited frequency f.sub.o .+-.2 MHz by a constant frequency according to each set mode, without regarding the relation between the currently tuned radio frequency signal frequency and the active broadcasting frequency. Thus, in the worst case, the search process of 64 steps is carried out to tune the broadcasting frequency, requiring an extensive amount of time to execute the frequency tuning process.